There are normal LT bus stops with 'head stop' and 'tail stop' on, and some even have the cheek to add 'driver information only'. What do these mysterious instructions tell the drivers?
Just a guess but head stop would mean the driver should stop the bus with the front next to the stop and tail stop the back of the buss level with the stop. This could be the case in london where some stops are capable of holding 2-3 busses at a time so they could both stop at the same time.
Its over 20 years since I lived in London, so I might be wrong about this, but I seem to remember that a head stop was where a route partially ended. For instance, I used to live in SE London and the 161 went from Woolwich to Chislehurst or Petts Wood. I think one of the stops in Chislehurst was marked as a head stop to remind the driver that this was where his route terminated (and he had to turn round) if he was not timetabled to go as far as Petts Wood. Its something like that.